Electric vehicles such as electric and hybrid automobiles include a high-voltage system that generates high voltages, for example 60 V DC or higher. A high-voltage line in electric vehicles including such a high-voltage system is designed typically in such a manner that the high-voltage line is insulated from the chassis ground. If the insulation resistance between the high-voltage line and the chassis ground decreases below a predetermined value, damage such as an electric shock to the human body can be caused depending on the position where the vehicle contacts the human body. Therefore, the insulation resistance between the high-voltage line and the chassis ground needs to be detected and, if an insulation resistance drop is found, warning needs to be given to a user to prompt the user to have the vehicle appropriately serviced.
A ground fault detecting circuit that is an apparatus for detecting the insulation resistance between a high-voltage line and a chassis ground is disclosed in Japanese Patent Laid-Open No. 2005-114497. FIG. 1 shows a configuration of the ground fault detecting circuit.
Referring to FIG. 1, the ground fault detecting circuit is a circuit detecting a ground fault from high-voltage DC power supply 100 to a body of a vehicle in an electric automobile driving circuit that includes high-voltage DC power supply 100 including batteries, DC/AC converter 101 including an inverter for converting an output from high-voltage DC power supply 100 to an AC current, and AC motor 102 supplied with an output from DC/AC converter 101. The ground fault detecting circuit includes oscillation circuit 103 outputting an AC signal (rectangular pulse) and voltage level detector 104 supplied with an output from oscillation circuit 103 through detection resistance 107, wherein a connection point P of oscillation circuit 103 and voltage detector 104 is connected onto a positive bus line of high-voltage DC power supply 100 by means of coupling capacitor 105 so that a DC component is blocked.
In the ground fault detecting circuit described above, a voltage appears at connection point P that results from division of a rectangular pulse from oscillation circuit 103 by insulation resistance 106 and detection resistance 107. In normal operation, the value of insulation resistance 106 can be assumed to be infinite and therefore the voltage at connection point P is substantially equal to the level of an output from oscillation circuit 103. If an insulation failure occurs and insulation resistance 106 drops, the voltage at connection point P significantly decreases. Therefore, an insulation failure can be found by detecting a change in the voltage at connection point P. Voltage level detector 104 determines that a ground fault has occurred between the negative bus line of high-voltage DC power supply 100 and the body of the vehicle if the voltage value at connection point P decreases below a reference voltage.